Carbon nanotube heat-exchange systems

ABSTRACT

A carbon nanotube heat-exchange system ( 10 ) and method for producing the same. One embodiment of the carbon nanotube heat-exchange system ( 10 ) comprises a microchannel structure ( 24 ) having an inlet end ( 30 ) and an outlet end ( 32 ), the inlet end ( 30 ) providing a cooling fluid into the microchannel structure ( 24 ) and the outlet end ( 32 ) discharging the cooling fluid from the microchannel structure ( 24 ). At least one flow path ( 28 ) is defined in the microchannel structure ( 24 ), fluidically connecting the inlet end ( 30 ) to the outlet end ( 32 ) of the microchannel structure ( 24 ). A carbon nanotube structure ( 26 ) is provided in thermal contact with the microchannel structure ( 24 ), the carbon nanotube structure ( 26 ) receiving heat from the cooling fluid in the microchannel structure ( 24 ) and dissipating the heat into an external medium ( 19 ).

CONTRACTUAL ORIGIN OF THE INVENTION

[0001] The United States Government has rights in this invention underContract No. DE-AC36-99GO10337 between the U.S. Department of Energy andthe National Renewable Energy Laboratory, a division of Midwest ResearchInstitute.

TECHNICAL FIELD

[0002] This invention relates to heat-exchange systems and morespecifically to carbon nanotube heat-exchange systems.

BACKGROUND ART

[0003] Most power-generation systems produce heat as a by-product. Forexample, internal combustion engines used to power most vehicles todaycombust a high-energy fuel (e.g., gasoline) to generate mechanicalmotion and heat. Fuel cells that convert hydrogen and oxygen intoelectricity and heat are also being developed for a variety ofapplications, including power production for vehicles and electricalappliances. Other power-generation systems, such as bio-fuel processing,petroleum refining, industrial processing, and solar-thermal systems, toname a few, also produce heat as a by-product. At least some of the heatproduced by such power-generation systems must be dissipated to theambient environment.

[0004] Various cooling systems have been developed for dissipating heat.Automobiles, for example, may have as many as fourteen separate coolingsystems, including cooling systems for the engine, oil, air conditioningsystem, and transmission. By way of illustration, most internalcombustion engines are cooled by a liquid (e.g., water, antifreeze) thatis circulated through a cooling loop provided in thermal contact withthe engine. As the liquid is circulated, it absorbs heat generated bythe fuel combustion. The cooling loop is connected to a heat-exchangesystem (e.g., a radiator). One type of automobile radiator may have atube arranged in a parallel or serpentine manner among a series ofcopper or aluminum “fins” that are provided in thermal contact with thesurrounding air. As liquid from the cooling loop flows through the tube,heat is conducted from the liquid into the air flowing past the fins(e.g., as the automobile moves).

[0005] The specific design and performance of currently availableheat-exchange systems is dominated by the heat transfer characteristicsof the materials from which these systems are made and convective heattransfer conditions on the fin surfaces. For example, typical automobileradiators may be fabricated from metals which have a relatively highthermal conductivity (e.g., aluminum, copper, etc.). However, thesematerials make the heat-exchange systems heavy, which negatively impactsthe automobile's performance, fuel consumption, and emissions. Recentstudies have shown that every twenty pounds-mass (lbm) of weight incurrent light-duty automobiles increases fuel use by 0.1 miles pergallon (mpg). In addition, typical heat-exchange systems have relativelyhigh air intake or air loading requirements so that the liquid can beeffectively cooled by the air flow. These loading requirements increasethe surface area that must be exposed to the air flow, making theheat-exchange system large and cumbersome. Indeed, radiators aretypically positioned at the front of the vehicle to maximize air flow tothe radiator. Consequently, these loading requirements also increasedrag on the automobile, negatively impacting the automobile'sperformance, fuel consumption, and emissions.

[0006] Other materials have also been studied for use with heat-exchangesystems. For example, carbon foams and porous ceramics (e.g., siliconcarbide) are highly conductive. Although these materials arelight-weight and exhibit relatively high thermal-exchange properties,these materials are structurally weak. Therefore, widespread use ofthese materials in heat-exchange systems is unlikely, especially inheat-exchange systems used on-board automobiles.

[0007] Consequently, a need remains for a high performance heat-exchangesystem that is structurally sound and light-weight. Additionaladvantages would be realized if the surface area and/or frontal loadingof the heat-exchange system were reduced. Fuel cell systems may also beimproved if the heat-exchange system can be used to cool one or morecomponents of the fuel cell directly.

DISCLOSURE OF INVENTION

[0008] Carbon nanotube heat-exchange system may comprise a microchannelstructure having an inlet end and an outlet end, the inlet end providinga cooling fluid into the microchannel structure and the outlet enddischarging the cooling fluid from the microchannel structure. At leastone flow path may be defined in the microchannel structure, fluidicallyconnecting the inlet end to the outlet end of the microchannelstructure. A carbon nanotube structure may also be provided in thermalcontact with the microchannel structure, the carbon nanotube structurereceiving heat from the cooling fluid in the microchannel structure anddissipating the heat into an external medium.

[0009] A method for producing a carbon nanotube heat-exchange system maycomprise the steps of fabricating a microchannel structure for receivinga cooling fluid, fabricating a carbon nanotube structure, and arrangingthe microchannel structure in thermal contact with the carbon nanotubestructure.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Illustrative and presently preferred embodiments of the inventionare shown in the accompanying drawings in which:

[0011]FIG. 1 is a high-level diagram illustrating a cooling system inwhich a heat-exchange system may be used according to one embodiment ofthe invention;

[0012]FIG. 2 shows a detailed section of one embodiment of theheat-exchange system wherein the carbon nanotubes are embedded in apolymer binder;

[0013] FIGS. 3(a) through 3(d) are transmission electron microscopy(TEM) images of carbon nanotube material that may be used to produce theheat-exchange system according to one embodiment of the invention;

[0014]FIG. 4 shows a detailed section of another embodiment of aheat-exchange system wherein the carbon nanotube structure comprisescarbon nanotubes grown directly on the microchannel structure;

[0015]FIG. 5 shows a detailed section of another embodiment of aheat-exchange system wherein the carbon nanotube structure is anopen-cell porous media; and

[0016]FIG. 6 shows a detailed section of another embodiment of aheat-exchange system for use with a fuel cell.

BEST MODES FOR CARRYING OUT THE INVENTION

[0017] Carbon nanotube heat-exchange system 10 (FIG. 1) and method forproducing the same is shown and described as it may be used in a coolingsystem 12 according to preferred embodiments of the invention. Briefly,heat-exchange systems 10 dissipate heat produced at a heat source 14(e.g., an internal combustion engine). A cooling fluid may be circulatedthrough a coolant loop 16 in and/or around the heat source 14 so thatthe fluid absorbs heat from the heat source 14. The heat-exchange system10 is provided in thermal contact with the fluid circulating through thecoolant loop 16 and with an external medium 19 (e.g., air). As thecooling fluid flows through the heat-exchange system 10, heat istransferred from the cooling fluid to the external medium. The coolingfluid may then be recirculated through the cooling loop 16 or dischargedto the environment. Alternatively, the heat-exchange system 10 of thepresent invention may be provided in direct contact with the heat source14, particularly where the heat source 14 is a relativelylow-temperature heat source. The particular design of the heat-exchangesystem 10 can impact the efficiency of the power-generation systems,particularly when used in vehicles. Therefore, it is desirable toproduce a structurally sound, high performance heat-exchange system.

[0018] A carbon nanotube heat-exchange system 10 (FIG. 2) may beproduced according to one embodiment of the invention as follows. Amicrochannel structure 24 may be fabricated having a flow path 28defined therein which fluidically connects an inlet end 30 (e.g., anintake manifold) to an outlet end 32 (e.g., a discharge manifold). Inone embodiment, the microchannel structure 24 may be extruded frommetal, although other embodiments are also described herein. A carbonnanotube structure 26 is also fabricated from carbon nanotubes. Forexample, the carbon nanotube structure 26 may be fabricated fromsingle-walled carbon nanotubes (SWNTs) 15 (FIG. 2) blended with apolymer to form a SWNT-polymer composite. In any event, the carbonnanotube structure 26 is arranged in thermal contact with themicrochannel structure 24 in such a manner so as to dissipate heat to aflowing medium (e.g., gas, air, or liquid) surrounding the SWNT-polymercomposite structure.

[0019] A carbon nanotube heat-exchange system 10 is shown in FIG. 2according to one embodiment of the invention comprising microchannelstructure 24 and carbon nanotube structure 26. At least one flow path 28fluidically connects an inlet end 30 of the microchannel structure 24 toan outlet end 32 of the microchannel structure 24. Carbon nanotubestructure 26 is arranged in thermal contact with the flow path 28 ofmicrochannel structure 24 and is also provided in thermal contact withan external medium, as illustrated by arrows 19.

[0020] In use, cooling fluid circulates through the coolant loop 16 ofthe cooling system 12 as illustrated by arrows 20, 21. The cooling fluidis introduced into the inlet end 30 of the microchannel structure 24 andflows through flow path 28 before being discharged from theheat-exchange system on outlet end 32. Heat is transferred from thecooling fluid flowing through the microchannel structure 24 to thecarbon nanotube structure 26, which in turn transfers the heat to theexternal medium (e.g., air) surrounding the carbon nanotube structure26. The cooling fluid may then be recirculated through the coolant loop16 to absorb more heat from the heat source 14. In other embodiments,the cooling fluid may be otherwise collected or released from thecooling system 12 (e.g., into the environment).

[0021] A significant advantage of the invention is the efficiency of theheat-exchange system 10. The relatively small flow paths 28 defined inthe microchannel structure 26 provide a thin thermal boundary layer forhighly-efficient two-phase or liquid-phase heat transfer. As such, themicrochannel structure 24 provides efficient heat transfer from thecooling fluid to the carbon nanotube structure 26. In addition, carbonnanotubes have demonstrated high directional or anisotropic thermalconductivity (e.g., in the range of about 3000 to 6000Watts/meter-Kelvin (W/m-K)). As such, the carbon nanotube structure 26provides highly efficient heat transfer to the external medium 19.Carbon nanotubes can also be fabricated into closely spaced structuresfor efficient convective heat transfer. The efficiency of theheat-exchange system 10 also allows for lower loading requirements forthe external medium. Accordingly, the heat-exchange system 10 of thepresent invention is compact and light-weight in design. When used withcooling systems for automobiles, the heat-exchange system 10 of thepresent invention reduces fuel consumption, lowers emissions, andincreases overall vehicle performance. In addition carbon nanotubes alsohave a very high elastic modulus (˜1 terra Pascal (TPa)), and can endurehigh critical strains (˜5%) before yielding, making the heat-exchangesystem 10 structurally sound and suitable for large-scale, commercialuse. The invention can also be adapted for use with fuel cells to coolone or more components of the fuel cell directly (e.g., heat-exchangesystem 210 in FIG. 4).

[0022] Having briefly described an embodiment of a carbon nanotubeheat-exchange system, as well as some of the more significant advantagesassociated therewith, various embodiments of the present invention willnow be described in greater detail below.

[0023] The heat-exchange system 10 may be used with any suitable coolingsystem, such as the cooling system 12 shown in FIG. 1. For example, theheat-exchange system 10 may be used with cooling systems for use withinternal combustion engines, bio-fuel processing, petroleum refining,industrial processing, and solar-thermal systems, to name only a few.Generally, the cooling system 12 has a coolant loop 16 that is inthermal contact with a heat source 14 (e.g., an internal combustionengine). One or more pumps 22 may be provided to circulate a coolingfluid through the coolant loop 16, as illustrated by arrows 20, 21. Notethat the cooling fluid will be referred to hereinafter as cooling fluid20. The cooling fluid 20 absorbs heat from the heat source 14(illustrated by lines 18) as it circulates in thermal contact with heatsource 14. The cooling fluid 20 is then delivered through the coolantloop 16 to heat-exchange system 10. The heat-exchange system 10transfers heat from the cooling fluid 20 to an external medium (e.g.,air from the ambient environment). Operation of the heat-exchange system10 will be explained in more detail below. The cooling fluid 20 may thenbe recirculated through the coolant loop 16 to absorb more heat from theheat source 14. Alternatively, the cooling fluid 20 may be dischargedfrom the coolant loop 16, collected for further processing, or otherwiseremoved from the coolant loop 16.

[0024] The coolant loop 16 may provide a flow path for the cooling fluid20 via any suitable conduits, such as rubber hoses, metal pipes, or PVCpipes, etc. Preferably, the conduits are made from, or coated with acorrosion-resistant material. In addition, the coolant loop 16 ispreferably sealed so that it does not leak cooling fluid 20.

[0025] The cooling fluid 20 that is circulated through the coolant loop16 may be any suitable liquid (e.g., water, antifreeze, etc.) or gas(e.g., air), and the external medium (illustrated by arrows 19) ispreferably an ambient medium (e.g., the surrounding air, water, etc.).Of course, the heat-exchange system 10 is not limited to use with anyparticular cooling fluid 20 or external medium 19. Any suitable coolingfluid 20, or two-phase fluid, and external medium 19 may be usedaccording to the teachings of the present invention and state-of-the artunderstandings in heat-transfer science, as will become apparent to oneskilled in the art after having become familiar with the teachings ofthe invention.

[0026] It should also be noted that the above description of the coolingsystem 12 shown in FIG. 1 is provided only as an illustration of oneenvironment in which the heat-exchange system 10 of the presentinvention may be used. The heat-exchange system 10, however, may be usedin conjunction with any suitable cooling system, now known or that maylater be developed. Furthermore, cooling systems, such as the one shownin FIG. 1, and modifications thereto are well-understood in the art ofheat-transfer science. Accordingly, the cooling system 12 will not bedescribed in further detail herein.

[0027] The heat exchange system 10 that may be used with cooling system12 to dissipate heat into the ambient environment according to oneembodiment of the invention may comprise a distribution manifold 30fluidically connecting the coolant loop 16 to a microchannel structure24. Cooling fluid 20 circulating through the coolant loop 16 flows intothe inlet end 30. In one embodiment, the inlet end 30 comprises adistribution manifold that disperses cooling fluid 20 among at least onemicrochannel 28 formed within the microchannel structure 24. A portion11 of the heat exchange system 10 is shown in more detail in FIG. 2according to one embodiment of the invention. Flow distribution amongthe microchannels 28 is illustrated by arrows 23.

[0028] The distribution manifold serves to disperse the cooling fluid 20from the relatively large coolant loop 16 (e.g., 5 to 10 centimeters(cm) in diameter) into the relatively small microchannels 28 (e.g., 1micron (em) to 1 millimeter (mm)). Preferably, the distribution manifoldis provided above or over the microchannel structure 24 so that thecooling fluid 20 flows in a downward direction into the microchannels28. Such an embodiment tends to more evenly disperse the cooling fluid20 from the coolant loop 16 into each of the microchannels 28. However,it is understood that other embodiments are also contemplated as beingwithin the scope of the invention, and indeed, other configurations arealso possible wherein the inlet end 30 is provided next to or even underthe microchannel structure 24. Likewise, the cooling fluid 20 may bepumped, pressurized, or simply flow by gravity.

[0029] The microchannel structure 24 may comprise one or more flow paths28 fluidically connecting the inlet end 30 to the outlet end 32 of themicrochannel structure 24. The cooling fluid 20 from the flow path 28 isdischarged from the heat-exchange system 10 on the outlet end 32. In oneembodiment, the outlet end 32 comprises a discharge manifold. Thedischarge manifold serves to collect the cooling fluid 20 (e.g., forreturn back into the cooling loop 16).

[0030] In one embodiment, the flow path(s) 28 in the microchannelstructure 24 may be characterized as being generally cylindrical inshape and cross-section and as having diameters that range from about 1micron (μm) to about 1 millimeter (mm). Such a design provides thinthermal boundary layers having relatively high heat transfercoefficients, especially when compared to the heat transfer coefficientstypical for larger, macro-scale flow paths. The higher heat transfercoefficients combined with an inherently large surface area provided bythe flow paths 28 for contact with the cooling fluid 20 serve toincrease the heat transfer capability of the microchannel structure 24.

[0031] For purposes of illustration, the section of microchannelstructure 24 is shown in FIG. 2 having six independent flow paths 28fluidically connecting the inlet end 30 to the outlet end 32. However,it is understood that the microchannel structure 24 may be fabricatedwith any suitable number of flow paths 28. For example, in anotherembodiment the microchannel structure 24 may comprise a single flow path28 formed therethrough. It is also understood that the flow paths 28 arenot limited to any particular geometry or size. Modifications can bemade to the microchannel structure 24 (and to flow paths 28 definedtherein) based on any number of design considerations, such as willbecome readily apparent to one skilled in the art of heat transferscience after having become familiar with the teachings of theinvention. Illustrative, but not exhaustive, of such designconsiderations are the volume of cooling fluid 20 provided to themicrochannel structure 24, the thermal conductivity of the material fromwhich the microchannel structure 24 is fabricated, properties of thecooling fluid 20 (e.g., density, viscosity, heat transfer coefficient,Prandtl number, etc.), and the amount of heat that is to be removed fromthe cooling fluid 20.

[0032] In addition, the microchannel structure 24 may be a heat pipe.According to such an embodiment, the carbon nanotube structure 26 maycomprise either carbon nanotubes “grown” directly on the heat pipeitself, or a polymer “superstructure” that is mounted thereto. Suchembodiments will be described in more detail below with respect to thecarbon nanotube structure 26. In any event, the carbon nanotubes may bearranged in any suitable manner on the heat pipe (e.g., on theevaporative portion, the transport portion, or the condensing portion).

[0033] The microchannel structure 24 may be fabricated using any of avariety of well-known manufacturing techniques. For example, themicrochannel structure 24 may be extruded or injection molded. Stillother manufacturing techniques, now known or that may be laterdeveloped, can also be used to fabricate the microchannel structure 24.

[0034] Generally, the microchannel structure 24 may be fabricated fromany suitable material. According to one embodiment, the microchannelstructure 24 may be fabricated from metal (e.g., aluminum, copper), ormetal alloys. However, other embodiments are also contemplated as beingwithin the scope of the invention. For example, the microchannelstructure 24 may be fabricated from plastic or ceramic. Yet otherembodiments are also contemplated as being within the scope of theinvention.

[0035] In another preferred embodiment, the microchannel structure 24,or portions thereof, may be fabricated from carbon nanotubes.Microchannel structures 24 fabricated from carbon nanotubes may reduceoxidation and fouling that may occur when the microchannel structure 24is fabricated from metal, and may therefore enhance the heat-transfercharacteristics of the microchannel structure 24. In one suchembodiment, single-walled carbon nanotubes (SWNTs) may be suspended in apolymer binder to form a SWNT-polymer composite. Production ofSWNT-polymer composites is explained in more detail below with respectto the carbon nanotube structure 26. The SWNT-polymer composite may thenbe injection molded or extruded to fabricate the microchannel structure24, or portions thereof.

[0036] The heat-exchange system 10 is also shown in FIG. 2 comprisingcarbon nanotube structure 26 arranged in thermal contact with themicrochannel structure 24. The carbon nanotube structure 26 ispreferably fabricated from single-wall carbon nanotubes (SWNTs).However, it is to be understood that in other embodiments the carbonnanotube structure 26 may be fabricated from multi-wall carbonnanotubes. The type of nanotubes used may depend on designconsiderations, such as the desired heat-transfer properties, cost ofmanufacture, among others.

[0037] For example, other design considerations include the so-called“percolation threshold”. That is, objects which are homogeneously loadedinto a matrix come into contact with one another as the density of theobjects in the matrix increases. The percolation threshold is defined asthe loading density where the objects are interconnected to form acontinuous pathway through the matrix. The density of objects requiredto reach the percolation threshold will depend on the size and shape ofthe objects as well as their tendency to agglomerate. Objects that arelong and thin are more likely to reach this percolation threshold atrelatively low loading levels.

[0038] Both multi- and single-walled carbon nanotubes are long andnarrow and the ratio of their length to width is typically in excess ofa factor of 10² and has been shown to exceed 10⁷. Thus, the percolationthreshold for these materials tend to be much lower than, for example,carbon black loading. The thermal conduction characteristics of anynanotube composite are expected to be superior above the percolationthreshold, and it is desirable that this threshold be reached with theminimum amount of high thermal conductivity material.

[0039] SWNTs can basically be described as nano-scale cylinders ofgraphite. A TEM image of raw, as-produced SWNT material is shown in FIG.3(a). The diameters and atomic arrangements of the SWNTs are dictated bythe geometric constraints that limit how a two-dimensional graphenelattice can be rolled to form a seamless tube. Individual SWNTs may havea diameter in the range of about 1 to 2 nanometers (nm) and a wallthickness of about 1 atomic carbon layer. The single atomic carbon layerfolds over into the shape of a long cylinder, thereby forming anindividual SWNT.

[0040] Two limiting SWNT structures are defined by the circumferencebeing comprised of sp2 bonded carbon atoms in either an “arm-chair” or a“zig-zag” configuration. Different types of arm-chair and zig-zagconfigurations with different diameters are also possible, as areconfigurations between these two limits having other helicities. Theso-called (10,10) arm-chair tube has a non-zero density of states at theFermi energy and therefore has properties of a metal. The (17,0) zig-zagtube is a true semiconductor with an energy gap. Calculating the densityof states for arm-chair tubes as a function of tube diameter shows thateach spike in the density of states is associated with an E^(−1/2)singularity characteristic of the dispersion in a one-dimensionalelectron conductor. These materials have a theoretical thermalconductivity as high as 6000 W/m-K. The diameters and helicities of theSWNT material can be controlled through synthesis.

[0041] In addition to the high thermal conductivity of SWNTs, SWNTs alsohave a demonstrated elastic modulus on the order of 1 TPa and cansustain critical strains of 5% before yielding. In addition, SWNTs are arelatively light-weight material. The high strength and small mass ofSWNTs creates mechanical resonant frequencies of 100 megahertz (MHz) to10 gigahertz (GHz). Accordingly, the carbon nanotube structure 24 iswell-suited for use with embodiments of heat-exchange system 10 of thepresent invention.

[0042] Carbon nanotube material may be generated by any of a number ofprocesses for use with the heat-exchange system 10 of the presentinvention. For example, carbon nanotube material may be generated usinglaser-based synthesis, growth by chemical vapor deposition (CVD) onmetal particles, solar furnace evaporation, and hot-wire deposition. Useof particular methods for generating carbon nanotube material is amatter of a design choice. Design considerations may include, but arenot limited to, cost, production quantities, types of nanotubes,interface bonding characteristics, heat-exchange system configurations,and the desired purity of the carbon nanotubes.

[0043] During production of the carbon nanotube material, metalparticles, graphite, and/or amorphous carbon may be formed along withthe carbon nanotube product from the raw carbon soot used to generatethe carbon nanotubes. Non-nanotube particulate matter provide sites forthe agglomeration of nanotubes, minimizing their effective homogenousdistribution in polymer solutions. Accordingly, it may be desirable topurify the carbon nanotubes before using them to fabricate the carbonnanotube structure 26 for the heat-exchange system 10. Any of a varietyof purification methods may be used that have been developed forremoving metal particles, graphite, and/or amorphous carbon from thecarbon nanotube product. A TEM image of 98 wt % pure SWNTs is shown inFIG. 3(b).

[0044] The carbon nanotube structure 26 may be fabricated from thecarbon nanotube material according to any suitable method now known orlater developed. For example, the carbon nanotube material may besuspended in a polymer binder. Techniques have been developed forcombining carbon nanotubes into a series of non-ionomeric polymersincluding polyethylene, poly-methyl methacralate (PMMA), polypropylene,polyacroylonitrile (PAN), polytetraflouroethylene (PTFE). Conductivepolymers may also be used to enhance the thermal characteristics of thenanotube-polymer composite. The carbon nanotube structure 26 may then befabricated from the suspended nanotube-polymer composite using anysuitable method, such as but not limited to, extrusion techniques orinjection molding.

[0045] The following describes an example of one technique that has beenused to generate a SWNT-polymer composite. First, the SWNT material wasblended into an ethanol/water solution that contained 5% weight forweight (w/w) of a perfluoro-polyester sulfonic acid ionomer (e.g.,Nafion (EW=1100)) and a 5 to 40% w/w aqueous polyester sulfonic acidionomer (e.g., Eastman AQ (EW=1000)). SWNT material was placed in thesolution and mechanically blended for about 72 hours at about 25° C. Thesolution was then centrifuged for 30 minutes at about 10,000 revolutionsper minute (rpm). The resulting supernatant was a homogenous solution ofSWNTs and ionomer.

[0046] The solution of SWNTs and ionomer was then solution cast as amembrane on a Teflon-coated aluminum template at 30° C., and formed amembrane of SWNT-polymer composite. The membranes were dried in vacuofor about 1 hour at about 80° C. to remove solvents and anneal theSWNT-polymer composite above the glass transition temperature (Tg) ofthe ionomer. The resultant films were then stored in a desiccator underargon. A TEM image of a SWNT-polymer composite membrane producedaccording to the example just described is shown in FIG. 3(c).

[0047] The SWNT-polymer composite membranes may be evaluated using avariety of spectroscopic, thermal, and mechanical analyses. For example,four point direct current (DC) resistivity measurements showed that theresistivity of an initial dry Nafion polymer is reduced to 200 Ohm-cmwith just 0.1% w/w loading of SWNTs. It is noted that good electricalconductivity is a strong indicator of good thermal conductivity. Asanother example, differential scanning calorimetry studies of a 1% w/wSWNT doped sample showed an increase in the glass transition of Nafionpolymer of about 20° C. at a heating rate of 10° C./min. Thermalgravimetric analysis (TGA) of the air oxidation of the Nafion polymershowed an increase to the onset of decomposition by 12° C. for the 1%SWNT-doped sample. These results indicate significant thermal andelectronic properties of SWNT-polymer composites, even those having verylow concentrations of SWNT material. Of course other analyses are alsopossible to characterize the heat-exchange properties of theSWNT-polymer composites, such as but not limited to, Raman spectroscopy,and UV-VIS-NIR spectroscopy to establish type and orientation ofnanotubes in the matrix.

[0048] Yet other properties of the SWNT-polymer composite may becontrolled during synthesis to produce SWNT-polymer composites havingdifferent thermal and mechanical properties. For example, thedirectional alignment of the SWNTs within the polymer matrix may becontrolled to produce bundled or aligned SWNT-polymer composites (seeFIG. 3(d)). One such technique for aligning SWNTs includes the use ofelectrical fields (electrophoresis) during extrusion or polymer castingof the SWNT material with polymeric substrates, such as polyethylene orPTFE. During such synthesis, the SWNTs align within the electricalfield. Other methods for producing different thermal properties includeattaching nanotubes having various functional groups to other polymersystems and then co-extruding them into a single fibrous co-polymer. Theheat transfer characteristics of the SWNT-polymer composite may also beenhanced by changing the density of the SWNT material, and the type ofpolymer material that is used, among other techniques.

[0049] The SWNT-polymer composite may be fabricated as one or more“fin-ike” structures to form a SWNT-polymer superstructure (i.e., carbonnanotube structure 26). In an exemplary embodiment, these fin-likestructures may each be about one-quarter to about three-eighths inchtall and about one inch wide. Of course other embodiments are alsocontemplated as being within the scope of the invention, and theparticular dimensions of the fin-like structures will depend at least tosome extent on various design considerations. In any event, the carbonnanotube structure 26 is arranged in thermal contact with themicrochannel structure 24.

[0050] According to one embodiment, the SWNT-polymer superstructure(i.e., carbon nanotube structure 26) may be bonded directly to themicrochannel structure 24. Techniques for attaching the carbon nanotubestructure 26 to the microchannel structure 24 include, for purposes ofillustration, metallurgical bonding (e.g., where the microchannelstructure 24 is made from a metal), use of a commercially availablebinder material (e.g., a metal or polymer binder), sintering, hot press,and electrochemical bonding techniques, to name a few. Alternatively,the carbon nanotube structure 26 may comprise carbon nanotubes “grown”directly on the microchannel structure 24, for example, using chemicalvaporization deposition (CVD) techniques. Such an embodiment is shown inFIG. 4, wherein two-hundred series reference numbers are used toidentify like-elements (e.g., microchannel structure 224). In yetanother embodiment, the microchannel structure 24 may comprisecorrugations upon which the carbon nanotubes are “grown” thereon.

[0051] It is noted that the carbon nanotube structure 26 is not limitedto having the fin-like structures that are shown in FIG. 2. The carbonnanotube structure 26 may be any suitable shape (e.g., rectangular,cylindrical, trapezoidal, etc.) and may be arranged on the microchannelstructure 24 in any suitable manner. The particular configuration maydepend at least to some extent on various design considerations, such asthe cross-sectional area, pressure drop requirements, and heat-transferrequirements.

[0052] According to one embodiment, the carbon nanotube structure 26 isimpermeable to the external medium 19. That is, the external mediumflows around and between the carbon nanotube structure 26, asillustrated by arrows 19 in FIG. 2, but not through the SWNT-polymercomposite. Heat that has been transferred from the cooling fluid 20 tothe SWNT-polymer composite is transferred from the carbon nanotubestructure 26 to the external medium 19. In addition, the flow around thefin-like structures may cause the carbon nanotube structure 26 tovibrate. Vibration during use disrupts the boundary layer, causing theboundary layer to remain thin, and increasing the heat-transfercharacteristics of the carbon nanotube structure 26.

[0053] A portion 111 of another embodiment of the heat-exchange system10 is shown in FIG. 5. Again, carbon nanotube structure 126 is arrangedin thermal contact with microchannel structure 124. The cooling fluid 20is dispersed by the distribution manifold 30 (FIG. 1) among themicrochannels 128 formed in the microchannel structure 124, asillustrated by arrows 123 in FIG. 5. In this embodiment, however, thecarbon nanotube structure 126 may be fabricated as an open-cell, porousmedia structure that the external medium 19 can readily permeate, asillustrated by arrows 119 in FIG. 5.

[0054] In one such embodiment, the open-cell, porous media structure 126may be fabricated by forming the carbon nanotubes into a matrix orstructure that surround voids (see FIG. 5). For example, the carbonnanotubes may be formed into triangles, squares, pentagons, hexagons,octagons, dodecahedrons, etc. to form a superstructure of carbonnanotubes (i.e., carbon nanotube structure 126), or even a structure ofrandomly interconnected pores (e.g., carbon nanotubes 115 in FIG. 5).Such structures may be formed by shaping the polymer or the carbonnanotubes in another binder material, or even pressing the carbonnanotube material into such formations without using a binder material.

[0055] Again with reference to FIG. 5, the external medium 19 flowsthrough the open pores in the carbon nanotube structure 126 and absorbsand dissipates thermal energy released from the cooling liquid 20flowing through the microchannel structure 124. This embodiment allowsthe porous media to be tailored (e.g., by sizing the open-celldiameters) to maximize thermal convection heat transfer within theporous media.

[0056] As discussed above, the heat-exchange system 10 may be used inautomobile cooling systems (e.g., cooling system 12). The heat-exchangesystem 10 exhibits unique thermal exchange properties that allow it tobe produced with a compact design. In addition, the frontal flow areafor the external medium 19 may be decreased. These design advantagesserve to improve the automobile's performance, and to reduce fuelconsumption and emissions by reducing overall weight and aerodynamicdrag. Indeed, in some applications, the heat-exchange system 10 may evenbe provided on the side(s) of the vehicle rather than in front of theautomobile, further decreasing aerodynamic drag.

[0057] It should be noted that the heat-exchange system 10 of thepresent invention can be used in any of a variety of applications, andis not limited to use with internal combustion engines. For example, theheat-exchange system of the present invention can also be used with fuelcells. Fuel cells convert hydrogen and oxygen into electricity and heat.The electricity can be used to power motors (e.g., for vehicles),lights, or various stationary and portable electrical appliances (e.g.,PCs). One embodiment of the heat-exchange system 210 is described hereinand shown in FIG. 6 as it can be used with a proton exchange membrane(PEM) fuel cell. Of course the heat-exchange system can be used with anyof a variety of other fuel cell types and is not limited to use with PEMfuel cells. Likewise, the heat-exchange system can be used with othercomponents of the fuel cell.

[0058] Briefly, the PEM fuel cell may comprise an anode catalyst 50(i.e., the negative terminal), a cathode catalyst 52 (i.e., the positiveterminal), and a membrane 54. Hydrogen gas is supplied to the fuel cellthrough channel 56, as illustrated by arrow 58. When a hydrogen moleculecomes into contact with the anode catalyst 50, the hydrogen moleculesplits and forms two positively charged hydrogen ions and two electrons.The electrons are conducted by the anode catalyst 50 and can then beused in an electrical circuit (e.g., to power a motor or otherelectrical device). Oxygen gas (e.g., in the form of air) is alsosupplied to the fuel cell through the channel 60, as illustrated byarrow 62, where it forms two oxygen atoms. Each of the oxygen atomsprovides a negative charge that attracts the two hydrogen ions. Themembrane 54 conducts positively charged ions (i.e., the hydrogen ions)and blocks electrons. Thus, the hydrogen ions are conducted through themembrane 54 where they recombine to form water.

[0059] Thermal energy is generated during this process for the most partat the fuel cell electrode. Hydrogen is not necessarily distributedevenly through the channel 56 in conventional fuel cells. For example,concentrations are generally higher at the inlet end. In addition, thechannel 56 is generally rectangular-shaped to maximize hydrogen flow.However, this design may cause a pressure drop in the channel 56 and/ormixing of the reacted hydrogen and unreacted hydrogen. Thus, theunreacted hydrogen concentration may be higher or lower in differentareas of the channel 56. Such uneven distribution of hydrogen may cause“hot spots” to form at various positions along the channel 56.

[0060] According to one embodiment of the invention, a thermalmanagement layer 57 may be integrated directly into one or morecomponents of the fuel cell or channel 56. In one such embodiment, thethermal management layer 57 may be fabricated from a carbonnanotube-based material, such as the SWNT-polymer composite describedabove. The thermal management layer 57 may be a channel, as shown inFIG. 6, or the thermal management layer 57 may be formed without achannel.

[0061] The carbon nanotube material serves as a high-conductivity pathto dissipate heat that may be generated and reduce or altogethereliminate the occurrence of hot spots. The SWNT material is alsoadvantageous in that it serves to store hydrogen. Because at least someof the hydrogen is supplied from the thermal management layer itself,where a channel 56 is provided, it may be made smaller and/or ofdifferent geometries to improve flow characteristics of the hydrogengas, and in turn, reduce heat generated by the fuel cell. In yet otherembodiments, the SWNT material may be charged with hydrogen prior tooperation of the fuel cell. Such an embodiment may serve to reduceelectrode and transport losses by the elimination of diffusion layers.

[0062] Of course it is understood that the heat-exchange system 10 mayalso be used in any of a variety of other applications. For example, theheat-exchange system 10 may be used in various electronic applications,such as but not limited to personal computers (PCs). In such anembodiment, the thermal management layer may be fabricated as a “tape”positioned in direct contact with the heat source so that it wicks heataway from the heat source. Indeed, the thermal management layer may evenbe channeled or routed around heat-sensitive components or entire areas,similarly to routing wires on thin-film transistors. In otherapplications, the carbon nanotube structure 26 may also be selectivelyarranged, such as on the condenser portion or the evaporator portion ofa heat pipe.

[0063] It is readily apparent that the carbon nanotube heat-exchangesystem 10 according to embodiments of the invention exhibits uniquethermal exchange properties. The relatively low weight, small frontalflow area for the external medium, and relatively high heat-exchangecapacity make the heat-exchange system 10 particularly advantageous asan alternative to conventional heat-exchange systems, especially for usein automobile cooling systems. Consequently, the claimed inventionrepresents an important development in the field of heat-exchangesystems.

[0064] Having herein set forth preferred embodiments of the presentinvention, it is anticipated that suitable modifications can be madethereto which will nonetheless remain within the scope of the presentinvention. Therefore, it is intended that the appended claims beconstrued to include alternative embodiments of the invention exceptinsofar as limited by the prior art.

1. A carbon nanotube heat-exchange system, comprising: a microchannelstructure having an inlet end and an outlet end, the inlet end providinga cooling fluid into said microchannel structure and the outlet enddischarging the cooling fluid from said microchannel structure; at leastone flow path defined in said microchannel structure, said at least oneflow path fluidically connecting the inlet end to the outlet end of saidmicrochannel structure; and a carbon nanotube structure provided inthermal contact with said microchannel structure, said carbon nanotubestructure receiving heat from the cooling fluid in said microchannelstructure and dissipating the heat into an external medium.
 2. Thecarbon nanotube heat-exchange system of claim 1, wherein the inlet endcomprises a distribution manifold.
 3. The carbon nanotube heat-exchangesystem of claim 1, wherein the outlet end comprises a dischargemanifold.
 4. The carbon nanotube heat-exchange system of claim 1,wherein said carbon nanotube structure comprises carbon nanotubes growndirectly on said microchannel structure.
 5. The carbon nanotubeheat-exchange system of claim 1, wherein said carbon nanotube structureis fabricated from single-wall carbon nanotubes (SWNTs).
 6. The carbonnanotube heat-exchange system of claim 1, wherein said carbon nanotubestructure is fabricated from multi-wall carbon nanotubes.
 7. The carbonnanotube heat-exchange system of claim 1, wherein said carbon nanotubestructure is fabricated from a SWNT-polymer composite.
 8. The carbonnanotube heat-exchange system of claim 1, wherein said carbon nanotubestructure is impermeable to the external medium.
 9. The carbon nanotubeheat-exchange system of claim 1, wherein said carbon nanotube structureis an open-cell porous media
 10. The carbon nanotube heat-exchangesystem of claim 1, wherein the carbon nanotube structure comprisesnanotubes bundled to form superstructures surrounding a void space, saidnanotubes bundles as triangles, squares, pentagons, hexagons, octagons,dodecahedrons.
 11. The carbon nanotube heat-exchange system of claim 1,wherein the microchannel structure is fabricated at least in part frommetal.
 12. The carbon nanotube heat-exchange system of claim 1, whereinthe microchannel structure is fabricated at least in part from carbonnanotubes.
 13. A carbon nanotube heat-exchange system, comprising athermal management layer fabricated from carbon nanotubes, said thermalmanagement layer dissipating heat into an external medium.
 14. Thecarbon nanotube heat-exchange system of claim 13, wherein said thermalmanagement layer stores hydrogen for release during operation of a fuelcell.
 15. The carbon nanotube heat-exchange system of claim 13, whereinsaid thermal management layer is routed away from heat-sensitive areas.16. The carbon nanotube heat-exchange system of claim 13, wherein saidthermal management layer is in direct contact with a source of the heat.17. A carbon nanotube heat-exchange system, comprising carbon nanotubemeans for transferring heat and dissipating the heat into an externalmedium.
 18. The carbon nanotube heat-exchange system of claim 17,further comprising microchannel means for transferring the heat from acooling fluid to said carbon nanotube means.
 19. A method for using acarbon nanotube heat-exchange system, comprising: receiving a coolingfluid through a microchannel structure; transferring heat from thecooling fluid in the microchannel structure to a carbon nanotubestructure; and dissipating the heat from the carbon nanotube structureinto an external medium.
 20. A method for producing a carbon nanotubeheat-exchange system, comprising: fabricating a microchannel structurefor receiving a cooling fluid; fabricating a carbon nanotube structure;and arranging the microchannel structure in thermal contact with thecarbon nanotube structure.
 21. The method of claim 20, furthercomprising fabricating the microchannel structure from carbon nanotubes.22. The method of claim 20, wherein arranging the microchannel structurein thermal contact with the carbon nanotube structure comprises growingthe carbon nanotube structure on the microchannel structure.
 23. Thecarbon nanotube heat-exchange system of claim 20, further comprisingarranging said carbon nanotube structure on said microchannel structureto avoid heat-sensitive areas.
 24. The carbon nanotube heat-exchangesystem of claim 20, further comprising controlling pore size of saidcarbon nanotube structure during fabrication thereof.
 25. The carbonnanotube heat-exchange system of claim 20, further comprising aligningSWNTs in said carbon nanotube structure during fabrication thereof.